Active load clamp for a track-and-hold circuit

ABSTRACT

A method and apparatus are provided for tracking and holding a voltage. The method includes the steps of providing an external analog voltage on an input of the track and hold circuit during a tracking mode of the track and hold circuit, storing a representation of the sampled external voltage in a storage device of the track and hold circuit, blocking a signal path between the external voltage and the storage device during a holding mode of the track and hold circuit and clamping the input of the track and hold circuit to a predetermined voltage during the holding mode.

This application is a continuation-in-part of U.S. Provisional PatentApplication Ser. Nos. 60/472,076 and 60/472,081, both filed on May 20,2003.

FIELD OF THE INVENTION

The field of the invention relates to signal sampling devices and moreparticularly to track-and-hold circuits for analog-to-digitalconverters.

BACKGROUND OF THE INVENTION

Track-and-hold circuits are generally known. Such devices are used inconjunction with analog-to-digital converters (ADC) in samplingcircuits. Sampling circuits are typically used to sample relatively highfrequency input signals (e.g. in the megahertz range).

In effect, ADCs require a relatively constant input voltage during ameasurement cycle to obtain an accurate measurement. However, if aninput signal is varying, then the ADC cannot obtain accurate signalmeasurements. In order to solve this problem, track-and-hold circuitsare used in conjunction with ADCs in sampling circuits.

Track-and-hold circuits operate by capturing an analog input signalduring a first tracking mode and holding the captured signal at arelatively constant voltage during a second, holding mode while the ADCmeasures the captured signal. During the tracking mode, thetrack-and-hold circuit may couple the input signal to a capacitor of thetrack-and-hold circuit. During the track mode, the input signal chargesthe capacitor to a voltage equal to that of the input signal.

During a second, holding mode, a transistor of the track-and-holdcircuit blocks a signal path between the signal storage device and inputsignal. During the second mode, the track-and-hold circuit presents arelatively constant voltage to the ADC for signal measurement.

While track-and-hold circuits work relatively well, their performance isdegraded in certain situations involving rapidly varying signals. Forexample, a rapidly varying signal on a blocking transistor inherentlyresults in signal leakage through the blocking transistor. Because ofthe importance of track-and-hold circuits, a need exists for a method ofpreventing signal leakage through the blocking transistor.

SUMMARY

A method and apparatus are provided for tracking and holding a voltage.The method includes the steps of providing an external analog voltage onan input of the track and hold circuit during a tracking mode of thetrack and hold circuit, storing a representation of the sampled externalvoltage in a storage device of the track and hold circuit, blocking asignal path between the external voltage and the storage device during aholding mode of the track and hold circuit and clamping the input of thetrack and hold circuit to a predetermined voltage during the holdingmode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a sampling system in accordance with an illustratedembodiment of the invention;

FIG. 2, comprising FIG. 2A and FIG. 2B, depicts a schematic of a bufferamplifier and track and hold circuit of FIG. 1;

FIG. 3 depicts a schematic of a buffer amplifier of FIGS. 1 and 3; and

FIG. 4 depicts a schematic of the feed forward circuit of FIG. 2.

DETAILED DESCRIPTION OF AN ILLUSTRATED EMBODIMENT

FIG. 1 is a block diagram of a sampling circuit 10 in accordance with anillustrated embodiment of the invention. Included within the samplingcircuit 10 may be a buffer amplifier 14, a track and hold (T/H) circuit16 and an ADC 18. As used herein, a track and hold circuit means adevice that samples an analog voltage from an external source during afirst tracking state and maintains a relatively constant representationof that sampled voltage in an internal storage device during a holdstate.

FIG. 2 is a schematic that depicts the buffer amplifier 14 and T/Hcircuit 16 of FIG. 1. FIG. 3 is a schematic of the buffer amplifier 14.

Under the illustrated embodiment, the buffer amplifier 14 and T/Hcircuit 16 are provided with a clamping circuit 20 that clamps an inputof the T/H circuit 16 to a predetermined voltage during a hold mode ofthe T/H circuit 16. Clamping the input of the T/H circuit 16 to apredetermined voltage has been found to substantially improve theperformance and bandwidth of the sampling circuit 10.

In general, an analog input signal from a signal source 12 is convertedinto a differential output signal within the buffer amplifier 14 andtransferred to the T/H circuit 16 on outputs Out_p and Out_n of FIG. 3.Within the buffer amplifier 14 (FIG. 3) a pair of emitter-followertransistors Q0 and Q1 provide a first quasi-differential amplifier forthe input signal. An output of the first differential amplifier (Bef_nand Bef_p) are provided as an input to a second amplifier stage. Thesecond amplifier stage, provided by transistors Q5 and Q6, may bereferred to as a degenerated differential pair. Resistor RPPC0 andtransistor Q4 form a current source (sink). RPPC6, RPPC12 are emitterdegeneration resistors. Resistors RPPC4 and RPPC13 and transistors Q7and Q8 form a portion of clamping circuits 20 (as discussed in moredetail below).

In tracking mode, differential switch transistors Q3 and Q5 and Q16 andQ23 (FIG. 2) are “on” while transistors Q2 and Q6 and Q17 and Q22 are“off”, thus biasing the emitter-followers Q0 and Q8 in their normalquiescent current region. The output of the buffer amplifier 14 (Out_Pand Out_n) is applied to the bases of emitter-follower switches Q0 andQ8 of track and hold sub-circuit 32. During the tracking portion of thetrack and hold process, switches Q0 and Q8 function to impose a chargeon charge storage devices (e.g., capacitors C0 and C5) in directrelationship to the input voltage from the buffer amplifier (i.e., fromoutputs Out_p and Out_n).

During the hold portion of the track and hold process, the currentswitches cut off the current to the Q0 and Q8 emitter-followertransistors, float the emitters and hence the hold capacitor nodes. Atthe same time, the current switches sink current at the bases of Q0 andQ8 helping to discharge the base-emitter junctions rapidly so that the“on” to “off” transition of the Q0 and Q8 transistors is very fast. Thesink current is also pulled through the output impedance of the bufferamplifier and the resulting voltage drop on the bases ensures that Q0and Q8 are completely deactivated. Once switches Q0 and Q8 aredeactivated, they block a signal path between the buffer amplifier 14and the capacitors C0 and C5.

The total hold capacitance of the track and hold subcircuits iscomprised of parasitics and MIM capacitors C0 and C5. Current switchtransistors Q2, Q3, Q16 and Q17 realize current switching betweentrack-and-hold modes for Q0 while Q5, Q6, Q22 and Q23 perform thisfunction for Q8.

A pair of feed forward compensators cap_bjt8 partially cancel anyfeedthrough voltage error by cross-coupling the base-emitters of Q0 andQ8. A schematic of the feed forward compensators cap_bjt8 is shown inFIG. 4. The compensators provide a series/parallel combination ofjunctions that emulate the base-emitter capacitance of Q0 and Q8.

The buffer amplifier 14 and T/H circuit 16 are designed to operate inthe 4 GHz region. At these high frequencies, the tradeoff betweenbandwidth, linearity, hold-mode droop and hold-mode feed-through becomesmore severe. The maintenance of high linearity in the switchedemitter-follower transistors driving the hold capacitor requires highcurrent drive, which, in turn, requires large transistors Q0 and Q8. Inparticular, the base-emitter junction nonlinearity induces 3^(rd) orderproducts generated by the emitter-follower driving the hold capacitanceand which are proportional to (fC_(h)/I_(e))³ where I_(e) is theemitter-follower quiescent current, C_(h) is the hold node capacitance,and f is the signal frequency. This serves as an impetus to minimize thehold capacitance while maximizing the quiescent current, but thisaggravates hold-mode droop. The increased capacitive loading on thepreamplifier associated with large transistors Q0 and Q8 reducesbandwidth. An even more difficult issue arises from the fact that largeemitter-follower transistors result in a substantial off-state couplingcapacitance to the hold capacitor, contributing to substantial hold-modefeed-through. Hence, increased linearity in the switched emitterfollower is obtained at the expense of bandwidth, droop, and hold-modefeed-through.

Another issue that arises in the switched emitter-followertrack-and-hold design is that the current bias required to optimize theswitched emitter-follower transistors is not necessarily compatible withthat required to produce the proper track-to-hold transition cutoffvoltage in the preamplifier load resistors. This is found to be the casewhen the bandwidth of the input buffer is extended to high frequencieswith high operating current to maximize linearity. The low value ofinput buffer output load resistance needed to maintain a small outputnode time constant requires more current for the track-hold cutofftransition than would be optimal for the switched emitter-follower bias.

These issues may be addressed with two new features incorporated intothe switched emitter-follower topology. First, the track-mode quiescentcurrent/hold-mode cutoff current discrepancy is addressed withadditional current switches Q16 and Q17 and Q22 and Q23 that increasethe hold-mode cutoff current by as much as 50% while leaving theswitched emitter-follower quiescent current unchanged. This provides anadditional degree of freedom to optimize the switched emitter-followerstage operating point and linearity while simultaneously allowingoptimization of the bandwidth and linearity of the buffer amplifier.Second, as shown in the input buffer amplifier schematic of FIG. 3, anew diode clamping circuit comprised of diode connected transistors Q7and Q8 is utilized to radically lower the effective buffer amplifierload impedance during hold mode. The supply voltage to these diodes isestablished such that they are essentially off during track-mode. Inhold-mode they conduct a portion of the current that would normally flowthrough the load resistors creating a dynamic impedance that shunts theresistors and substantially lowers the buffer amplifier gain.Simulations show that this feed-through rejection feature provides anorder of magnitude improvement in feed-through levels relative to whatwould be obtained with the feed-through compensation capacitors bythemselves. This feature enables the track/hold circuit to operate at avery broad bandwidth with good linearity while maintaining lowfeed-through levels.

Returning now to the figures, in order to further eliminate voltagefeedthrough, clamping circuits 20, (operating under control of a set oftrack and hold control connections 22, 24) may be provided to activateand deactivate the switches Q0 and Q8. In this regard, substantiallyidentical clamp circuits 20 may be provided in the positive and negativesides of the signal path. For purposes of simplicity, the operation ofthe positive side of the clamp circuit 20 will be described. It shouldbe understood that the clamp circuit 20 on the negative side of thesignal path operates in substantially the same manner.

The clamping circuit 20 on the positive side may include a differentialcurrent switch assembly 26, an auxiliary current switch 28 (shown inFIG. 2) and a clamping diode assembly 30 (shown in FIG. 3). Thedifferential current switch assembly 26 may include transistors Q2 andQ3 and a constant (tracking) current source (sink) (i.e., transistor Q13and resistor RPPC0) 36 that sinks 8 mA.

During the tracking mode, the tracking control connection Clk_p is high(i.e., assumes a logical value of 1) and Clk_n is low (i.e., assumes alogical value of 0). The effect of the high value on Clk_p causestransistor Q3 to conduct and transistor Q2 of the differential currentswitch assembly 26 to be in an off state.

Similarly, the auxiliary current switch 28 may include transistors Q17and Q18 and a constant (helper) current source (sink) (i.e., transistorQ15 and resistor RPPC6) 34 that sinks 4 mA. During the tracking mode,transistor Q16 is in a conductive state and transistor Q17 is in an offstate.

During the tracking state, switch Q0 is biased into a linear portion ofits operating range by current switching transistor Q3 and the constantcurrent source. Because of its biasing characteristics, switch Q0 can beassumed to have an amplification factor of 1. With an amplification of1, the capacitor C0 very closely tracks the input voltage.

During the hold mode, the control connection Clk_p goes low and Clk_ngoes high. Due to this signal transition, transistors Q3 and Q16 becomedeactivated and transistors Q2 and Q17 becomes activated. Activation oftransistors Q2 and Q17 pulls the base of the switch Q0 downwards to apredetermined voltage determined by the clamping diode assembly 30 ofFIG. 3.

The diode assembly 30 shown in FIG. 3 may include a transistor Q8 andresistor RPPC13. RPPC13 may have a value of 139 ohms and be connectedbetween Vp (supply voltage of 5 volts) and Out_p.

Transistor Q8 has its base connected to its collector, thereby allowingthe transistor Q8 to function essentially as a diode. The transistor Q8is connected between Out_p and Vcl. Vcl may have a nominal voltage of3.7 volts.

During the tracking state, current flows through the resistor RPPC13,but does not flow through the transistor Q8. Limiting current flow tothe resistor RPPC13 provides a linear relationship between the drivingsignal (Bef_p-Bef_n) and the output signal on Out_p.

During the hold state, activation of transistors Q2 and Q17 (FIG. 2)pulls the connection Out_p low because current sources 34, 36 within thedifferential current switch 26 and auxiliary switch 28 sink a total of12 mA. The 12 mA is provided by the diode clamping assembly 30.

The base to emitter voltage of the diode of Q8 of FIG. 3 has been foundto be about 0.9 volts (for a silicon germanium hetero-junction bipolartransistor). With a voltage drop of 0.9 volts and a Vcl voltage of 3.7volts, the signal connection Out_p would assume a value of 2.8 volts.

With a Vp value of 5 volts and an Out_p value of 2.8 volts, the voltagedrop across RPPC13 is 2.2 volts. With a resistance value of 139.67 ohms,the current through RPPC13 would be approximately 15 mA.

The reader should note, in this regard, that the 15 mA through RPPC13 isthe total current being drawn by all devices. For example, the secondamplifier stage contains a current source (Q4, RPPC0) that sinks 16 mA.In a quiescent condition, RPPC13 would be expected to source one-half ofthat current, or 8 mA, to current source Q4. As a result, and during thehold mode, the 12 mA being drawn by the differential current switchassembly 26, and auxiliary current switch 28 would be divided betweenRPPC13 and diode Q8. Since the voltage across the diode Q8 is notcurrent dependent, 7 mA of the 12 mA would be drawn from RPPC13 and 5 mAof the 12 mA would be provided by the diode Q8.

Because diode Q8 in the hold mode has an incremental voltage change ofvirtually zero volts for an incremental change in current, the diode Q8also functions as a low impedance filter that reduces (filters out) anyinput signal from the source 12 in the second amplification stage byover 25 dB during the hold mode. The net result is that the clampingcircuit 20 clamps the input of the track-and-hold sub-circuit 32 to thepredetermined voltage of 2.8 volts and virtually eliminates the inputsignal during the hold mode.

The operation of the clamp diode and buffer amplifier can also bevisualized from the viewpoint of AC small signal operation. In the holdmode, the clamp diode is forward biased, producing a relatively small ACsignal impedance (5-10 ohms) that is much smaller than the value of theload resistor RPPC13. The resulting parallel combination of the resistorRPPC13 and the forward biased clamp diode provide an AC impedance thatreduces the total collector load impedance. Since the gain of the Q5/Q6differential pair is determined by the product of the differentialtransconductance and the collector load impedance, the significantreduction of the collector load impedance in the hold mode radicallyreduces the gain of the Q5/Q6 differential pair during the hold mode.This reduction in gain attenuates the signal, thereby providing asignificant reduction in the feed-through signal applied to the trackand hold sub-circuit 32 during the hold mode.

It should be noted that while the auxiliary current switch 28 performssubstantially the same function as that of the differential currentswitch assembly 26 in the hold mode, it would not be possible to combinethe two circuits. The reason is that the current sinking requirementduring the track mode is different than the current sinking requirementduring the hold mode.

In this regard, the transistor Q0 may be sized with a base region thatis proportional to the size of the capacitor C0. The current sink 36 isthen sized to allow the transistor Q0 to operate in a linear region ofits operating range. If the base region of the transistor Q0 wereinstead sized for the total current sinking requirements of the clampcircuits 20 (i.e., 12 mA), then the transistor Q0 would be much largerthan necessary. Making the base region of the transistor Q0 larger thannecessary would increase the base-to-emitter capacitance, therebyincreasing the feedthrough signal to the capacitor C0.

A specific embodiment of method and apparatus for operating a track andhold circuit has been described for the purpose of illustrating themanner in which the invention is made and used. It should be understoodthat the implementation of other variations and modifications of theinvention and its various aspects will be apparent to one skilled in theart, and that the invention is not limited by the specific embodimentsdescribed. Therefore, it is contemplated to cover the present inventionand any and all modifications, variations, or equivalents that fallwithin the true spirit and scope of the basic underlying principlesdisclosed and claimed herein.

1. A method of tracking and holding a voltage, such method comprisingthe steps of: providing an external analog voltage to an input of atrack and hold circuit during a tracking mode of the track and holdcircuit; storing a representation of the sampled external voltage in astorage device of the track and hold circuit during the tracking mode;blocking a signal path between the external voltage and the storagedevice during a holding mode of the track and hold circuit; and clampingthe input of the track and hold circuit to a predetermined voltageduring the holding mode.
 2. The method of tracking and holding a voltageas in claim 1 wherein the step of storing the representation furthercomprises coupling the input signal to the storage device through anemitter-follower transistor.
 3. The method of tracking and holding avoltage as in claim 2 further comprising coupling a collector of theemitter-follower transistor to a supply voltage and an emitter of theemitter-follower transistor to the storage device.
 4. The method oftracking and holding a voltage as in claim 3 further comprising couplinga tracking current sink in parallel with the storage device.
 5. Themethod of tracking and holding a voltage as in claim 4 furthercomprising defining the storage device as a capacitor.
 6. The method oftracking and holding a voltage as in claim 5 wherein the step ofblocking further comprises disconnecting the tracking current sink fromits parallel connection with the storage device.
 7. The method oftracking and holding a voltage as in claim 6 wherein the step ofclamping further comprises reconnecting the tracking current sink to abase of the emitter-follower transistor during the holding mode.
 8. Themethod of tracking and holding a voltage as in claim 7 wherein the stepof clamping further comprises coupling a helper current sink to the baseof the emitter-follower circuit.
 9. The method of tracking and holding avoltage as in claim 8 wherein the step of clamping the input to thepredetermined voltage further comprises coupling a filtering devicebetween a voltage reference and the input.
 10. The method of trackingand holding a voltage as in claim 9 wherein the step of coupling thefiltering device between the voltage reference and the input furthercomprises coupling a diode between a first voltage reference and theinput.
 11. The method of tracking and holding a voltage as in claim 10wherein the step of coupling the filtering device between the voltagereference and the input further comprises coupling a resistor coupledbetween the input and a second voltage reference.
 12. An apparatus fortracking and holding a voltage, such apparatus comprising: means forproviding an external analog voltage to an input of a track and holdcircuit during a tracking mode of the track and hold circuit; means forstoring a representation of the sampled external voltage in a storagedevice of the track and hold circuit during the tracking mode; means forblocking a signal path between the external voltage and the storagedevice during a holding mode of the track and hold circuit; and meansfor clamping the input of the track and hold circuit to a predeterminedvoltage during the holding mode.
 13. The apparatus for tracking andholding a voltage as in claim 12 wherein the means for storing therepresentation further comprises means for coupling the input signal tothe storage device through an emitter-follower transistor.
 14. Theapparatus for tracking and holding a voltage as in claim 13 furthercomprising means for coupling a collector of the emitter-followertransistor to a supply voltage and an emitter of the emitter-followertransistor to the storage device.
 15. The apparatus for tracking andholding a voltage as in claim 14 further comprising means for coupling atracking current sink in parallel with the storage device.
 16. Theapparatus for tracking and holding a voltage as in claim 15 furthercomprising defining the storage device as a capacitor.
 17. The apparatusfor tracking and holding a voltage as in claim 16 wherein the means forblocking further comprises means for disconnecting the tracking currentsink from the storage device.
 18. The apparatus for tracking and holdinga voltage as in claim 17 wherein the means for clamping furthercomprises means for connecting the tracking current sink to a base ofthe emitter-follower transistor during the holding mode.
 19. Theapparatus for tracking and holding a voltage as in claim 18 wherein themeans for clamping further comprises means for coupling a helper currentsink to the base of the emitter-follower circuit.
 20. The apparatus fortracking and holding a voltage as in claim 19 wherein the means forclamping the input to the predetermined voltage further comprises meansfor coupling a filtering device between a voltage reference and theinput.
 21. The apparatus for tracking and holding a voltage as in claim20 wherein the means for coupling the filtering device further comprisesa diode coupled between a first voltage reference and the input.
 22. Theapparatus for tracking and holding a voltage as in claim 21 wherein themeans for coupling the filtering devices further comprises a resistorcoupled between the input and a second voltage reference.
 23. A trackand hold circuit for tracking and holding a voltage, such apparatuscomprising: an external analog voltage coupled to an input of the trackand hold circuit during a tracking mode of the track and hold circuit; astorage device adapted to store a representation of the external analogvoltage during the tracking mode; a blocking circuit that blocks asignal path between the external voltage and the storage device during aholding mode of the track and hold circuit; and a clamping circuit thatclamps the input of the track and hold circuit to a predeterminedvoltage during the holding mode.
 24. The apparatus for tracking andholding a voltage as in claim 23 wherein the means for storing therepresentation further comprises a first emitter-follower transistorthat couples the input signal to the storage device.
 25. The apparatusfor tracking and holding a voltage as in claim 24 further comprising acollector of the first emitter-follower transistor coupled to a supplyvoltage and an emitter of the first emitter-follower transistor to thestorage device.
 26. The apparatus for tracking and holding a voltage asin claim 25 further comprising a tracking current sink coupled inparallel with the storage device.
 27. The apparatus for tracking andholding a voltage as in claim 26 further comprising defining the storagedevice as a capacitor.
 28. The apparatus for tracking and holding avoltage as in claim 27 wherein the blocking circuit further comprises asecond transistor that open circuits the tracking current sink.
 29. Theapparatus for tracking and holding a voltage as in claim 28 furthercomprising a third transistor that connects the tracking current sink toa base of the emitter-follower circuit during the holding mode.
 30. Theapparatus for tracking and holding a voltage as in claim 29 furthercomprises a helper current sink that is coupled to the base of theemitter-follower circuit during the holding mode.
 31. The apparatus fortracking and holding a voltage as in claim 30 further comprising afiltering device coupled between a voltage reference and the input tothe track and hold circuit.
 32. The apparatus for tracking and holding avoltage as in claim 31 wherein the filtering device further comprises adiode coupled between a first voltage reference and the input.
 33. Theapparatus for tracking and holding a voltage as in claim 32 wherein thefiltering devices further comprises a resistor coupled between the inputand a second voltage reference.